Digital color printers typically form images by applying varying amounts of different colorants to a receiver media. One common class of digital color printers that is used for many different applications is inkjet printers. A typical inkjet printer reproduces an image by ejecting small drops of ink from a print-head containing ink nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. Inkjet printers typically reproduce color images by using a set of color inks, usually cyan, magenta, yellow, and black. Different image colors can be formed by varying the number of ink drops for each of the different color inks that are printed in a local region of the image. The different spectral absorption characteristics of the inks enables the control of the spectral characteristics of the printed image, and therefore the color perceived by a human observer. For a given image spectra, the perception of color by a human observer can be characterized by the well-known CIE XYZ tristimulus values:X=∫R(λ)I(λ) x(λ)dλY=∫R(λ)I(λ) y(λ)dλZ=∫R(λ)I(λ) z(λ)dλ  (1)where R(λ) is the reflectance spectrum of the printed image as a function of the wavelength, λ; I(λ) is the spectrum of the light source illuminating the printed image; and x(λ), y(λ) and z(λ) are the CIE color matching functions, which are related to the spectral sensitivities associated with human vision. Often, the CIE XYZ values are transformed into other color spaces such as the well-known CIELAB color space, which are more visually uniform. The CIELAB color space has three color channels: L*, which is a measure of the lightness of the color; a*, which is a measure of the redness-greenness of the color; and b*, which is a measure of the yellowness-blueness of the color. The chroma of a color is a measure of its colorfulness. For the CIELAB color space, the chroma is defined by the relationship:C*=√{square root over (a*2+b*2)}.   (2)Visual color differences are often estimated by computing the Euclidean distance between two CIELAB color values. This quantity is referred to as ΔE*:ΔE*=√{square root over ((L*2−L*1)2+(a2*−a1*)2+(b2*−b1*)2)}{square root over ((L*2−L*1)2+(a2*−a1*)2+(b2*−b1*)2)}{square root over ((L*2−L*1)2+(a2*−a1*)2+(b2*−b1*)2)}  (3)
Color transforms are typically used with digital color printers to determine the amounts of each of the colorants that are necessary to accurately reproduce a desired input color. For example, if it is desired to make an inkjet print of an image that is displayed on a softcopy display, the input color (e.g., the CIELAB values) for a given image pixel can be determined by characterizing the color response of the display. The color transform will then determine the amounts of the colorants (e.g., cyan (C), magenta (M), yellow (Y) and black (K) inks) that should be used to produce the desired CIELAB values. For digital color printing systems having more than 3 colorants there are typically a plurality of different colorant combinations that can all produce the same CIELAB values. For example, consider the case where it is desired to produce a gray color with L*=50, a*=0, b*=0 on an inkjet printer having CMYK inks. It would be possible to produce the desired color using only the cyan, magenta and yellow inks and no black ink. On the other hand, it would be possible to produce the same color using only black ink, with no cyan, magenta and yellow inks. Alternatively, some intermediate level of the black ink could be used, together with some intermediate amount of cyan, magenta and yellow inks. The choice of which combination of ink amounts should be used is determined by the designer of the color transforms, taking into account a number of different factors such as the amount of ink that the media can absorb and the image granularity characteristics as a function of the ink amounts. For example, using more K and less CMY will generally use lower ink amounts and will therefore be less susceptible to artifacts such as coalescence. However, the graininess of images using larger amounts of K is usually higher due to the fact that the black ink drops are more visible to a human observer. The designer of the color transforms must balance these types of characteristics to determine the CMYK colorant amounts that will produce the highest overall image quality.
Another attribute that can impact the image quality of images produced on digital color printers is color inconstancy. Color inconstancy results when the color appearance of an image changes when it is viewed under different light sources. The cause for the artifact can be understood by considering Eq. (1). The XYZ values, and therefore the perceived color of the image, are a function of the product of the spectrum of the light source, I(λ), and the image reflectance spectrum, R(λ), together with the color matching functions. Depending on the shapes of these spectra, significantly different perceived colors may result when the light source is changed. For example, a patch in the image may appear to be neutral gray when observed under daylight illumination, but may appear to have a reddish tint when observed under tungsten illumination and a greenish tint when observed under fluorescent illumination. In some cases, the magnitude of the color shift may be quite noticeable, and depending on the image content may be quite objectionable. Generally, the color shifts are most objectionable in near-neutral and skin tone colors.
The magnitude of the color shifts associated with color inconstancy will be a strong function of the colorants used to produce a particular color. Generally, colorant combinations which produce image spectra having more distinct spectral peaks will be most susceptible to color inconstancy artifacts, particularly when viewed under light sources having sharp spectral transitions (e.g., fluorescent illuminants). A “process neutral” color produced by combining cyan, magenta and yellow inks will typically have a spectra with three distinct peaks associated with the absorption bands of the three inks. As a result, it will generally be much more sensitive to color constancy artifacts than a neutral color produced using black ink alone which would generally have a much flatter reflection spectra. Similarly, using a mixture of CMY and K inks will produce an intermediate level of color inconstancy.
The fact that colors produced using larger amounts of black ink will have substantially lower levels of color inconstancy artifacts would suggest that it would be desirable to use high levels of black ink in the design of color transforms for CMYK digital color printers. However, as mentioned earlier, there are a number of other factors that need to be considered. One of the most significant factors is the image granularity. The image granularity associated with the use of higher levels of black ink can significantly degrade the image quality due to the fact that the black ink dots are generally darker and more visible than cyan, magenta and yellow ink dots. Therefore, in the design of a color transform, it is necessary to balance color inconstancy artifacts with granularity artifacts and select the transform that would be preferred for the particular image.
U.S. Pat. No. 6,637,861 describes a method for building color transforms that determines the level of black ink usage by setting a limit on the acceptable color shifts induced by color inconstancy effects. A typical color shift limit is given to be ΔE*=4 across a defined set of illuminants. U.S. Patent application 2005/0111020 describes a variation of this method where black ink is only used in the color transform for darker colors. This has the effect of limiting the image granularity in highlight regions of an image where the black ink dots will be most visible.
U.S. Pat. No. 7,365,879 and U.S. Patent Application 2006/0250624 describe methods for building color transforms using a multi-dimensional cost function to determine an optimal balance between different attributes that affect image quality. A color inconstancy index is given as one attribute that can be included in the multi-dimensional cost function, together with other attributes such as an ink volume metric, an image noise metric, a lightfastness metric, a waterfastness metric, a gloss metric or a metameric index.
The optimal balance between color transforms that limit color inconstancy artifacts and those that limit other artifacts such as image granularity is generally quite image dependent. Grayscale images, as well as color images having a lot of near-neutral colors are particularly sensitive to objectionable color inconstancy effects. Therefore, for these types of images the optimal balance will typically limit color inconstancy artifacts by using higher levels of black ink while accepting somewhat higher levels of other artifacts such as image granularity. On the other hand, observers will generally be much more tolerant of color inconstancy artifacts for color images that are dominated by high chroma colors and have very little near-neutral image content. In these cases, the optimal balance will typically use lower levels of black ink in order to minimize artifacts such as image granularity.
It is well-known in the art that it may be desirable to use different color transforms for different types of image content. For example, different color print modes are often provided for color and grayscale images. The different print modes are generally selected by the user in the printer driver at the time that a document is printed. For example, U.S. Pat. No. 6,851,794 describes a method where a plurality of color transforms having different color inconstancy limits are determined, providing different compromises between color inconstancy artifacts and other types of artifacts. The user can select the transform that is most desirable for a particular image/application by making a selection in the printer driver. However, this method has the disadvantage that it requires the user to navigate through menus in the printer driver in order to access these benefits and has to understand what choice is best for different images. This can result in a very unsatisfying user experience.